What is the difference, do you think, between these two sentences: I am going to study for an exam tonight and I am going to be studying for an exam tonight. The two statements mean roughly the same thing, but close reading and careful thinking rarely settle with rough answers. The two sentences are not identical, so why?
Studying how a language works, even one’s own, is never a simple affair, and that is why it can be very helpful to organize our questions under the three traditional headings of grammar, logic, and rhetoric. Any statement we speak or write involves at once all three of these motives, and knowing what kind of question we are asking can simplify our investigation. If, when we ask about the difference between the two sentences in our example, we mean the grammatical difference, then we can proceed to analyze what phrases and clauses we find and explain the function of each. If, though, we want to know whether the bare meaning differs, that is a logical question. And if we are concerned to understand the effect the style of each would have on the listener or reader, then we are involved in a rhetorical pursuit.
To say as I did, then, that the two statements are roughly equivalent means that logically they are asserting the same thought: I will study for an exam tonight. But the first sentence says to study and the second, to be studying. If the two statements are logically the same, why this grammatical difference? First let’s understand how the grammar differs, and then let’s ask to what effect.
Each sentence is grammatically simple, which means each has only one independent clause: I am going. This subject-predicate combination is an idiomatic construction in English (and similarly so in other languages) in which the present tense and progressive aspect of the verb go is used to indicate an action very soon to occur in the future. Dependent on this main clause is then an infinitive phrase, to study in the first sentence, and to be in the second. These infinitives make up a regular part of this peculiar future construction; it’s what happens after the infinitives that changes how the same thought is received.
In the first sentence, a prepositional phrase follows the infinitive to study, and we should classify that prepositional phrase as an adverb because it has something to do with the infinitive, a verbal structure. To say that I am going to study for an exam is to explain the reason for my studying. Elements that answer the question why? or to what purpose? are by definition adverbs, and so this first sentence intends to say what it means in as bare-bones a fashion as possible, it seems. The second version, by contrast, retains this same prepositional phrase but has associated it with the participle studying, not the infinitive to study. That one change changes the effect in an interesting and subtle way.
Participles are adjectives built from verbs, which means they serve to describe some active, or dynamic, quality of a noun or pronoun. In the second example, the main clause I am going to be means that the subject pronoun I will exist in a certain way in the near future, and the participle that follows indicates exactly how: one who will be studying. The speaker or writer has chosen here to draw out the action of studying in order to characterize the subject for some reason. Such a choice is a rhetorical, or stylistic, concern, and without a context, we can only surmise the purpose. Perhaps it was to contrast the action of studying with another action someone else was proposing, to go out for dinner, for example.
When we are concerned to arrange our grammar and logic like this to bring about a certain effect on a listener or reader, we are orchestrating the three arts of language, grammar, logic, and rhetoric, in the way those instruments were meant to sound together. To do that, and to understand why, is the full experience.